Terminal Motility in Elongating Stolons ofProboscidactyla flavicirrata

Abstract

Proboscidactyla flavircirrata is a commensal hydroid whose stolons elongate rapidly in the absence of its host. Growth pulsations comparable to those of other hydroids originate endogenously in each stolon tip, although influenced by the to and fro movements of the hydroplasm. Neither growth pulsations nor elongation appear to depend upon cellular proliferation, because the distal ends of stolons lacked mitotic figures and since irradiation sufficient to halt mitosis failed to stop stolon elongation. Cellular migration is also apparently unessential for elongation because severed stolon tips, isolated from potential sources of reusable cells, nevertheless elongated. Marking experiments suggested that cells changed shape rather than location as severed tips advanced. In old preparations the stolons became attenuated because the tips continued to progress without adequate recruitment of cells. Stolons deprived of their tips failed to elongate although they were still connected to the remainder of the colony and contained mitotic cells. Cytochalasin B simultaneously halted growth pulsations and stolon elongation in a reversable manner. Apparently stolon tip locomotion in Proboscidactyla is essential to the elongation process.